The principles of operation, design and fabrication of electric micromotors have been investigated by numerous parties in recent years. Good background information and further discussion of the invention is contained in various of the following references, which are hereby incorporated by reference in their entireties:
W. Trimmer and R. Jebens, "An Operational Harmonic Electrostatic Motor", Proc. of Micro Electro Mechanical Systems, IEEE Robotics and Automation Council, Salt Lake City, Utah, Feb. 20 -22, 1989, at 1-16;
M. Mehregany et al, "Microfabricated Harmonic and Conventional Side-Drive Motors," Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Mass., Feb. 1990.
M. Mehregany et al, "A Study of Three Microfabricated Variable-Capacitance Motors," Proc. 4th Int. Conf. on Solid State Sensors and Actuators (Transducers '89), Montreux, Switzerland, June 25-30, 1989;
S. C. Jacobsen et al, "The Wobble Motor: An Electrostatic, Planetary Armature, Microactuator," Proc. of Micro Electro-Mechanical Systems, IEEE Robotics and Automation Council, Salt Lake City, Utah, Feb. 20-22, 1989, at 17-24
M. Mehregany et al, "Principles in Design and Microfabrication of Variable Capacitance Side-Drive Motors," 36th National Symposium of the American Vacuum Society, Boston, Mass., Oct. 23-24, 1989, at 108.
Mehregany et al., "Friction and Wear in Microfabricated Harmonic Side-Drive Motors," presented at IEEE Solid State Sensor & Actuator Workshop, Hilton Head, S.C., June 4-7, 1990.
Mehregany et al., "Operation of Microfabricated Harmonic and Ordinary Side-Drive Motors," presented at 3rd IEEE Workshop on Micro Electro Mechanical Systems, Napa Valley, Calif., February 1990.
Broadly speaking, it is the goal of workers in this field to produce machines or systems composed of large numbers of extremely small subsystems, each comprising one or more micromotors. One article describes such micro electro mechanical systems (MEMS) as machines constructed of small moving sub elements which have characteristic dimensions in the range of about 0.5 to 500 microns. The motivation for the development of such devices is the possibility of extending the advantages of small scale, which are currently available in electronic devices, to include mechanical systems with moving parts. Actuation systems composed of numerous, small elements could benefit from the ability to achieve high local field strength in regions between very small force producing elements, from using geometrically complex structures to arrange, and parallel, the force producing elements, such that they closely interact across large interfacial areas, and by combining the outputs of these structures in appropriate parallel-series combinations in order to develop desired relationships between force, displacement and velocity.
In general, micromotors are conceived to be electrostatic devices which must meet a number of challenging design constraints. The generation of strong electrostatic forces from reasonable voltages requires that gaps (across which fields act) be extremely small, of submicron dimensions if possible. Friction becomes a crucial consideration for relative motion of surfaces across such gaps. Relative surface smoothness is a much worse problem for microscopic surfaces than for macroscopic surfaces. Friction therefore can be a substantial factor. A destabilizing tendency is also present in any electrostatic device, as conductive or dielectric elements tend to clamp to any electrically active element. In micromotors fabricated to date, it is the rotor which tends to become clamped to an underlying shield. This rotor clamping has been attributed to the existence of a electric field between the rotor and the shield, which field is caused by a lack of proper electrical contact between these respective parts. The electrical contact between the rotor and the shield during motor operation is intended to come from mechanical contact of the rotor with the shield or the bearing, all of which are heavily doped polysilicon components. To facilitate this contact, the bearing often is fabricated in electrical contact with the shield. The electrostatic clamping of the rotor is a significant problem with variable-capacitance side drive micromotors. To overcome the frictional forces associated with the clamping of the rotor to the electric shield beneath it, air levitation assistance has been employed. However, it is desired to avoid the need for air levitation.
One of the potentially useful motor designs for micro actuators is the wobble motor, which is already reported in the literature. See, for example the Jacobsen et al article. Wobble motors have desirable feature, such as a built-in gear reduction and good torque characteristics, but so far they have suffered from the aforementioned need for air levitation assistance.
With reference to FIG. 1, which shows the geometry of a conventional wobble motor 10 in cross-section, the motor comprises a stator assembly 12 and a rotor 14. The rotor 14 contacts the stator at a point 16, called the rolling point (RP). The rotor axis 18 moves in a circular path around and parallel to the stator axis 22, at the wobble frequency .omega..sub.w. As the rotor moves, the rolling point also moves around the interior aperture in the stator. At steady state, the wobble frequency is equal to the frequency .omega..sub.s at which the stator segments 24.sub.1 -24.sub.N are cycled. The rotor output frequency, .omega..sub.r, is related to the wobble frequency by the relationship .omega..sub.r =.omega..sub.w [1-(R.sub.2 /R.sub.r)], where R.sub.r represents the outer radius of the rotor and R.sub.s represents the inner radius of the stator. The rotor output frequency can thus be much slower than the wobble. In other words, there is an inherent gear ratio between the electric signal frequency and the rotor frequency.
Note that insulation is required on the rotor, and/or stator in direct contact during motor operation. This complicates the manufacturing process.
Five different fabrication schemes for wobble motor design are discussed in S. C. Jacobsen et al. These five approaches are: (1) direct micro assembly (DMA); (2) electro-discharge machining (EDM); (3) cylindrical photographic etching methods (CPE); (4) coextrusion of metal and plastic (CMP) and (5) silicon chip stack of from one to many chips, with etched holes aligned to form the stator cavity (SCS). None of these fabrication schemes is particularly well suited to the efficient, low-cost mass manufacture of reliable micromotors. Although these motors do not have a clamping problem since they do not utilize a substrate or consequently a shield, they are not compatible with integrated circuits for fabrication or interface purposes.
It is therefore an object of the present invention to provide an improved design for a micromotor.
Another object of the invention is to provide an improved design for a microfabricated wobble motor.
Yet a further object is to provide a micromotor formed of a suitable, semiconductor material.
Another object of the invention is to provide a micromotor which will operate for extended intervals without the need for air levitation.
Yet another object is to provide a wobble-type micromotor which does not require insulation on the rotor.
A further object of the invention is to provide a wobble micromotor which can be fabricated using integrated circuit manufacturing techniques.